The amplification of electrical signals to produce amplified acoustic audio signals is well known in the art. Common applications where signals are amplified and provided to speakers to produce acoustic signals include telephone systems and public address systems.
In a public address system, an acoustic audio signal is received by a microphone, converted to an electrical signal, amplified by an amplifier, and provided to a speaker where it is reproduced as an amplified acoustic audio signal. In many situations, a portion of the amplified acoustic audio signal is received by the microphone. Because the electrical signals received by the microphone are, in effect, the same signals previously provided to the amplifier, a feedback loop is established, where the feedback loop includes both electrical and acoustic coupling. Oftentimes, the microphone in a public address system is located very near the speakers of the system. Depending upon the dynamics of the speakers, the microphone, the gain of the amplifier, and the acoustics of the room or space in which the system resides, positive feedback may result causing large audible acoustic signals at particular frequencies. As one skilled in the art will readily appreciate, the physical dimensions of the room, the relative positioning of the microphone and the speaker, the gain of the amplifier, and the density of the air will determine at which particular frequencies feedback occurs.
In older hands-free telephone systems, half-duplex, or one-way, communication was used to eliminate feedback. While one user was talking, reception from the other user was not allowed. Thus, no feedback loop could be established. Full-duplex telephone systems, however, are forced to contend with the feedback problem. In some cases, the relative positioning of the speaker and microphone is fixed to reduce feedback. In such systems, probable feedback frequencies can be determined, and in some cases the system can be designed to include filtering apparatus to attenuate any feedback that may occur at these probable feedback frequencies.
With the advent of full-duplex hands-free telephone sets where the speaker is in a fixed location and the microphone moves, the relative positioning between the microphone and the speaker changes as the microphone moves. Thus, the acoustic coupling between the microphone and the speaker also changes. For this reason, it is difficult to anticipate at which frequencies feedback may occur in the system, thus making preventative filtering impractical.
Acoustic feedback suppression systems in public address systems are known in the art. For example, the acoustic feedback suppression system disclosed in U.S. Pat. No. 4,079,189 uses an analog filtering technique for conditioning signals prior to their amplification and coupling to the speaker. The prior-art system employs a plurality of analog filters within the signal path to attenuate signal components that appear to contain feedback. The device selectively tunes the analog filters to increase or decrease the attenuation based upon the particular feedback behavior of the system. The analog circuitry required for this system, however, is both expensive and complex. Further, this analog system suffers the shortcoming of inaccuracy in determining the bandwidths and attenuation levels of the filters.
Other prior-art solutions digitize the audio information and process the resulting digital audio signal in order to remove unwanted feedback. These solutions perform a time-to-frequency conversion on the digital audio signal using algorithms such as the Fast-Fourier Transform in order to obtain the frequency spectrum of the signal. The frequency spectrum can then be examined for spikes or areas of high magnitude that represent feedback. The signal, in digital or analog form, can then be filtered to remove the feedback components. Because of the processing power required to implement algorithms such as the FFT, multiple processors may be necessary to convert to the frequency domain, detect the feedback, and filter the signal to remove the feedback. Single processors having a large amount of processing power may be able to support such a system, but the amount of processing power consumed when implementing the FFT leaves little power for other signal processing functions that may be desired.
Therefore, a need exists for a method and apparatus for efficient detection and removal of feedback components in audio systems, where the frequencies of feedback components may change over time.